This invention relates generally to multiple trace triggerable oscilloscopes and more particularly to a circuit for simultaneously displaying the trigger signal and two input signals.
Conventional triggerable oscilloscopes generally can be selectively triggered from either an external trigger input (external), the power line input (line) or one of the normal input signals (internal). The selected trigger signal initiates a horizontal sweep when its amplitude passes through a pre-selected trigger level threshold. Since the sweep generator has an inherent time delay from the time a trigger is produced until the display actually starts on the cathode-ray tube screen, the display signal is deliberately delayed to allow the leading-edge portion of a rapidly changing waveform to be displayed.
In addition, conventional triggerable oscilloscopes contain binary electronic switching logic which allows two normal input signals to be simultaneously displayed in a chop or alternate mode. The chop mode is normally clocked by a chop oscillator and the alternate mode by the detection of the end of the horizontal sweep. Rapid switching between the two signals results in a simultaneous display due to the `memory` of the phosphors in a conventional cathode-ray tube.
Certain known circuits also allow the user to replace the normal input channel display with a display of the trigger signal alone. Such a circuit is described in U.S. Pat. No. 3,795,834 entitled, OSCILLOSCOPE HAVING EXTERNAL TRIGGER DISPLAY MODE issued on Mar. 5, 1974 to Roland Eugene Andrews et al. These circuits result in the undesired loss of the input display when the trigger signal display is selected. This is particularly disadvantageous to a user when it is desired to make subtle timing measurements between the trigger signal and the signals input to channels A and B. These critical timing measurements require the simultaneous display of both the trigger signal and the two input signals. An example would be the determination of the small time changes resulting from an adjustment of the trigger level threshold. These circuits are also incapable of continuously monitoring the trigger signal while concurrently displaying input signals. A user would also desire to view the trigger signal exactly as applied to the trigger recognition circuit so that he can choose a trigger level threshold that avoids the likelihood of triggering on noise or other waveform anomalies. In addition, it is desirable to be able to identify the precise point on the trigger signal that initiates the sweep.
Accordingly, the principal objects of this invention are to provide a display mode for simultaneously displaying the trigger signal and the input signals, in correct time relationship, for displaying the trigger signal exactly as applied to the trigger recognition circuit, and for identifying the point on the trigger signal that initiates the sweep. Other and incidental objects of the present invention will become apparent from a study of the following detailed description.
These objects are accomplished in accordance with the preferred embodiment of the invention by electronically multiplexing the trigger signal and the input signals on channels A and B when either the alternate or chopped display is selected and the sweep trigger signal display logic is activated, to produce a three channel display. In either chopped or alternate display mode the electronic switching logic is transformed from a binary counter to a ternary counter upon activation of the trigger signal display, which allows the multiplexing of the three signals resulting in a simultaneous three channel display.